As the integration density and/or speed of metal oxide semiconductor field effect transistors (MOSFETs) has continued to increase, various processes have been studied to try increase the performance and/or reliability of the transistors. Particularly, many processes have been developed to try increase mobility of electrons or holes in order to produce high-performance transistors.
A process of applying physical stress to a channel area to change an energy band structure of the channel area may be performed to increase the mobility of the electrons or the holes. For example, NMOS transistors may have improved performance in the case of when tensile stress is applied to a channel, and PMOS transistors may have improved performance in the case of when compressive stress is applied to a channel. Accordingly, a dual stress film structure has been proposed where a tensile stress film is formed on the NMOS transistors and a compressive stress film is formed on the PMOS transistors, to allow improved performances of both the NMOS transistors and the PMOS transistors.